1. Field of the Invention
The present invention relates to a secondary radar transponder comprising an antenna arrangement which can receive interrogation signals from all directions and which radiates reply signals.
2. Description of the Prior Art
In secondary radar operation, for example, in the military field for identification friend or foe (IFF), all possible types of targets, e.g. land craft, aircraft, ships, are interrogated from fixed or mobile radar positions. For this purpose, these targets must have transmitting-receiving installations, i.e. a so-called transponder, having a suitable antenna arrangement.
Such an antenna arrangement must be able to receive interrogation signals from all directions and retransmit reply signals. Depending upon the type of identification system, the receiving signal (interrogation) and the transmitting signal (reply signal) can lie in adjacent frequency ranges or in frequency ranges which are far removed from one another. In a particular type of interrogation, for example, the frequency of the receiving signal is determined by the primary radar frequency of the interrogation station. Since primary radar apparatus can function with very different frequencies, the transponder antenna, therefore, must be able to receive over an extremely large frequency bandwidth, for example, over several octaves.
The polarization of the transmitting signal and the receiving signal of the transponder antenna arrangement can be the same, for example, vertical, or orthogonal relative to one another, for example, vertical/horizontal.
For an identification system having narrowly adjacent frequency ranges for receiving and transmitting (1030/1090 MHz) it is known to employ an omnidirectional antenna (unipole antenna or annular gap antenna). For receivers and transmitters in different frequency bands, most frequently, integrated-designed double antennas having omnidirectional patterns are employed. Such antennas are known in the art; for example, reference may be taken to the German published application Nos. 2,354,550 and 2,629,502.
In all known cases, the transponder antenna retransmits the reply signal, however, by means of an omnidirectional radiation pattern. This signifies an unnecessary scattering of the radiated energy which results in a low antenna gain and, in the presence of many interrogated objects, in an enormously strong radio room load. The radio room load causes a superposition of the reply signals and reciprocal interference, as a consequence of which the interrogation safety is reduced or the interrogation is prevented altogether. An additional disadvantage of the radiation of the reply signals from the target transponder by means of an omni-directional radiation pattern is the danger of the target being detected by enemy reconnaissance systems.